A proposal for developing a medical expert system shell for application in various areas of medicine (internal medicine, laboratory medicine, psychology, etc.) that is based on a new method for evidential reasoning-is advanced. This system can be applied as: (1) a screening system for detecting risk patients in the hospital, the out-patient department, and the practitioner's office (2) a laboratory diagnostic system for automatically generating laboratory reports with diagnostic summaries, and (3) a sophisticated medical consultation system for differential diagnosis. Through the newly developed evidential reasoning method that is based on Dempster-Shafer Theory of evidence, an adequate representation of medical entities and relationships between them is possible. Specifically, this method allows us to capture the uncertainty in the definition of disease, symptoms, laboratory test results, and in their mutual association. The expert system shell will be tested in the field of inflammatory joint diseases (arthrites), an area for which our Co- Principal Investigator has both computerize medical knowledge and patient records.